SAN DIEGO (CBS 8) - There was an added police presence Friday at all San Diego Unified schools that received threats Thursday, according to school representatives and they stressed that while the officers were not visible, they were present.

In addition to the added security, police are conducting an investigation to determine who is responsible for the threats that triggered lockdowns at 11 local high schools.

The first local campus to receive a menacing call was San Ysidro High, which was phoned about 9:15 a.m. by someone who made a vaguely ominous claim that "something was going to happen" there soon, said Manny Rubio a spokesman for Sweetwater Union High School District. Officers searched the classrooms and grounds, then gave an all-clear about 11 a.m., he said.

Then starting at about 9:30 a.m., threats about shootings or explosives were received at 10 San Diego Unified School District campuses: Clairemont, Crawford, Kearny, La Jolla, Mira Mesa, Morse, Patrick Henry, Point Loma, San Diego and Scripps Ranch high schools, SDUSD spokeswoman Ursula Kroemer reported.

Most of the calls were received within a roughly 20 minute period, and several others occurred later in the morning, according to Kroemer.

Administrators put the campuses on lockdown while San Diego police conducted sweeps. District officials advised parents that there was no need to go to any of the campuses to pick up students.

The searches were complete and all the lockdowns lifted by early Thursday afternoon. Kroemer said. There were no reports of anything out of the ordinary being found at any of the schools.

Students were let out of classes at the usual times, according to the spokeswoman.

School representatives will not say if a student was involved in these threats or if the threats are related. Police said they hope to have an update and an arrest sometime next week.